The present invention relates to a noise detection circuit and, more particularly, to a digital noise detection circuit for detecting the presence of noise in digital diphase data and for producing an alarm signal in response to the detection of such noise.
In present day communications and switching systems, especially those requiring a large number of precisely-timed operations, it is important that the timing of these operations be properly and correctly executed even in the presence of noise or other similar high-energy disturbances. By way of example, it is common in many digital communications and switching systems to provide a master timing generator from which all timing signals for the system are derived and which is phase locked to an external clock signal to insure that all of the timing signals derived from the master timing generator are in proper, precise time and phase relationship. The external clock signal which is employed by the master timing generator for phase locking purposes may be conveniently derived in one common approach by extraction from digital data such as digital diphase data having binary information encoded therein by means of standard conditioned diphase modulation techniques. If the external clock signal is corrupted by noise or other high-energy disturbances, the master timing generator will attempt to track the erroneous signal, thereby causing timing perturbations within the system and, consequently, causing serious synchronization problems within the system and degradation of the performance of the system. It is important in the above situation therefore that the presence of noise be quickly and readily detected so that the operation of the system can be altered during the presence of the noise or other appropriate measures taken to compensate for the presence of the noise.
Techniques employed heretofore for detection of noise in digital signals have been essentially analog in nature, employing analog filters and comparators. These analog components are susceptible to thermal effects and variations in component tolerances which can adversely affect the stability and accuracy of the noise detection circuitry. In addition, these components are relatively slow in operation and lack the level of sensitivity required to meet stringent high-frequency specifications of equipment such as master timing generators. Further, if it is desired to change the frequency of clock signals such as described hereinabove, it is necessary to reprogram the values of the analog components.